1. Field of the Invention
The present invention relates to a method and apparatus for displaying an image signal, and, more particularly, to a method and apparatus for reducing color error bands occurring due to the abrupt difference in brightness between sub-pixels in an apparatus for displaying an image signal, such as a liquid crystal display (LCD), a plasma display panel (PDP), a light-emitting diode (LED), or an organic light emitting diode (OLED).
2. Description of the Related Art
A color space is a model for representing color numerically in terms of three or more coordinates. Different image processing systems employ different color spaces for different reasons. At present, most apparatuses for displaying an image signal, such as a color CRT monitor, LCD monitor, or a PDP monitor, use the RGB color space.
The RGB color space uses the three primary colors red (R), green (G), and blue (B), which can be added to form other colors. Spectral components of these colors are added to one another to make other colors.
An RGB model includes a three-dimensional cube whose corners are red, green and blue, its origin is black, and the opposite end of the three-dimensional cube is white. For example, in a 24-bit color system having 8 bits per color, red is denoted by (255,0,0).
The RGB model makes design of a computer graphic system simple, but it is not ideal for every application. This is because of relations among R, G and B color components. Various image processing techniques such as histogram equalization depend on the brightness of an image. Accordingly, RGB images need to be frequently converted into brightness images. In order to convert images from RGB colors into a brightness level, Equation (1) is used (National Television Systems Committee (NTSC) standard).Y=0.288R+0.587G+0.114B  (1)
Among R, G and B components, since the G component acts as a significant component in a brightness level, only the brightness corresponding to green is used as data for the brightness level. Consequently, the G component is generally positioned in the center of a pixel in a display of a sub-pixel structure; sub-pixels are arranged in the order of R, G and B, or B, G and R.
The image display technique based on the aforementioned RGB sub-pixels may be divided into a technical field for enhancing resolution and a technical field for correcting a pixel error. Examples of enhancing resolution include a technique for generating image data using a controller in pixel and sub-pixel driving modes as discussed in U.S. Pat. No. 6,823,088 and a technique based in a re-sampling filter as discussed in U.S. Patent Application No. 2004/0080479.
Examples of correcting a pixel error include a technique for sub-sampling an image after magnifying the image three times in a state where a database for font is maintained as discussed in U.S. Pat. No. 6,756,992, and a technique for removing jagging of text as discussed in U.S. Patent Application No. 2003/0214513.
The present invention belongs to the aforementioned technical field of correcting the pixel error but has aspects and solutions different from those of the aforementioned technical fields.
FIG. 1 illustrates a character “A” displayed by an RGB sub-pixel type display. As shown in FIG. 1, one pixel has a structure in which a plurality of RGB sub-pixels 11 is alternately arranged in a space. If each of the RGB sub-pixels 11 has a pixel value of a certain size, all the colors can be displayed by combination of the pixel values of the sub-pixels. A black pixel 12 constituting a character row can be displayed as RGB sub-pixels having values of 0, 0 and 0. However, since the RGB sub-pixels respectively occupy different spaces, a problem occurs in that exact sub-pixel blending may not be provided. Particularly, this problem is most noticeable in the case of a large-sized pixel or at the boundary between pixels. For example, blue may be displayed at the left side of a character row by a blue sub-pixel positioned at a left boundary of the black pixel 12.
FIG. 2 illustrates color error bands. As shown in FIG. 2, color error bands 21 and 22 may occur where a brightness value is abruptly varied by perceptive characteristics, such as a boundary between objects. A distorted result different from the strength of an actual image signal is caused by the color error bands 21 and 22. These color error bands are most noticeable when the pixel is large.